FIELD OF THE INVENTION
The present invention relates to a thyristor having current sources of opposite polarity connectible thereto, and to a method for operating such a thyristor.
Of the several pn-junctions of a conventional thyristor, only the central pn-junction separating the two base layers from one another is polarized in the non-conducting direction in the inhibited or non-conducting state of the thyristor, such that substantially the entire voltage across that junction is removed. The positive holes are thereby displaced from that portion of the p-base adjacent to the central pn-junction. At the same time, the negative electrons are displaced from the adjacent portion of the n-base. The immobile acceptors and donors of the doping material thus produce a space charge zone at the central pn-junction. The voltage at the respective pn-junctions between the emitters and the adjacent base layers decrease by a negligible amount, so that no emission can occur into the base layers from the emitters.
When electrons from the n-emitter and holes from the p-emitter are injected into the respectively adjacent base layers, these continue to dissipate the space charge zone. The inhibiting effect of the central pn-junction is dissipated to the same degree. As a result, the voltages at the remaining pn-junctions decrease by a substantially larger amount and intensify the emission of the emitters. Finally, the injected charge carriers, in the form of a quasi-neutral plasma, flood the two base layers and the central pn-junction. The voltages at all of the pn-junctions do not noticeably change thereafter; the thyristor is fully triggered and now conducts a load current which is essentially determined by an external circuit (load circuit) and the load resistor therein.
If the external circuit is an ac circuit the thyristor is quenched at the next successive zero axis crossing of the pending voltage. If the external circuit is a dc circuit, or when the thyristor in an ac circuit is to be quenched before the zero axis crossing is reached, the inhibited condition must be reintroduced; this being accomplished by eliminating the electron/hole plasma flooding the central pn-junction.